The anode of a typical solid electrolytic capacitor consists of a porous anode body with a lead wire extending beyond the anode body and connected to the positive mounting termination of the capacitor. The anode is formed by first pressing a valve metal powder into a pellet. Alternatively, the anode may be an etched foil, for example aluminum foil as is commonly used in the industry. The anode is a conductive material, more preferably a metal or conductive metal oxide or conductive metal nitride. A particularly preferred anode material is a valve metal, conductive oxide of a valve metal or a conductive nitride of a valve metal with the preferred valve metals including Al, Ta, Nb, Ti, Zr, Hf, W, and mixtures thereof. Al, Ta, Nb and NbO are particularly preferred anode materials. The pressed anode is sintered to form fused connections between the individual powder particles. All anodes are anodized to a pre-determined voltage in a liquid electrolyte to form an oxide of the valve metal which serves as the dielectric of a solid electrolytic capacitor. A primary cathode material, such as a conductive polymer or manganese dioxide, is subsequently applied. Application of primary cathode material is commonly via a liquid dipping process, but other methods such as spraying, brushing, rolling, etc. can also be employed. In order to minimize the equivalent series resistance (ESR) of solid electrolytic capacitors a highly conductive coating, usually silver paint, is applied to the surfaces of the device. Similar to the primary cathode material application methods include dipping, spraying, brushing, etc. A carbon layer, usually applied between the primary cathode material and terminal silver layer, serves as a chemical barrier to isolate the two layers. The silvered anodes are then assembled onto a carrier material that commonly provides the end terminations, and the assembly is encapsulated to form the finished devices. The encapsulation process may be a transfer molding process, or conformal coating process, to manufacture surface mount capacitors. Conformal coating with a plastic sealant is often used to manufacture leaded devices. The industry standard end product shape for surface mount capacitors are rectangular parallelepiped, or cuboid, thus the anodes used in these devices are similarly shaped to maximize volumetric efficiency of the device.
In hermetically sealed capacitors the silvered anodes are placed in cylindrical cans containing a solder plug. Heat is applied to the can to reflow the solder. After reflow the solder secures the anode in place and forms an electrical connection between the cathode and the metallic can. The anodes used in these devices are cylindrical.
The reliability of all such devices is highly dependent on the quality of the external cathode layers.
Dielectric formation is never perfect, and results in leakage sites between the anode and cathode materials. The ability to isolate flaws in the dielectric is a requirement of the primary cathode material chosen for manufacturing solid electrolytic capacitors. This property of the primary cathode material results in a process termed ‘healing’. The application of voltage to the capacitor causes current to flow through flaw sites in the dielectric, resulting in an increase in the temperature at the defect site due to Joule heating. The temperature of the cathode layer immediately adjacent to the flaw site increases due to conduction. As current flows through the flaw site, the counter electrode material immediately adjacent to the flaw site is rendered nonconductive. When manganese dioxide is employed as the cathode material, the manganese dioxide immediately adjacent to the flaw site is converted to manganese sesquioxide at the decomposition temperature of manganese dioxide (500-600° C.), thus isolating the flaw. Since the resistivity of manganese sesquioxide is several orders of magnitude greater than that of manganese dioxide, leakage currents through the flaw sites decrease according to Ohm's law. A similar mechanism is postulated for conductive polymer counter electrodes. Possible mechanisms to account for the healing mechanism of conductive polymer films include complete decomposition of the polymer adjacent to the flaw site, over oxidation of the polymer, and dedoping of the polymer at the flaw site. At temperatures above 600° C. the amorphous tantalum pentoxide which serves as the dielectric in tantalum capacitors is converted to a conductive crystalline state. Thus, in order to be an effective primary cathode material for tantalum the material must convert to a nonconductive state at temperatures below 600° C. The maximum withstanding temperatures of other valve metal oxides is similar to that of tantalum.
Since the graphite and silver layers do not decompose to form nonconductive materials at temperatures below 600° C., continuous coating of all dielectric surfaces by the primary cathode material is essential to prevent the graphite or silver layers from contacting the dielectric. If the graphite or silver do contact the dielectric there will be a short circuit of the device.
Conductive polymer coatings are applied to the anode using a variety of methods as described in U.S. Pat. No. 6,072,694. The use of polymer slurries or liquid suspensions containing pre-polymerized conductive polymer as an alternative to the monomer is very attractive due to the simplicity of manufacturing, the reduction in waste, and the elimination of costly and time consuming washing steps after each coating step as directed in U.S. Pat. No. 6,391,379. Although this process approach is attractive, it has not yet been implemented on a production scale. One of the principle technical obstacles to the successful implementation of a polymer slurry to serve as the primary cathode layer is the difficulty coating edges and corners of the anode with slurry. These materials tend to pull away from corners and edges due to surface energy effects. The resulting thin coverage at corner and edges results in reduced reliability of the device. The magnitude of the force pulling the liquid away from the edge is given by the Young and Laplace Equation:Δp=γ/r Wherein                Δp=the pressure difference causing the liquid or slurry to recede from an edge        γ=the surface tension of the liquid or slurry; and        r=the radius of curvature of the edge.This effect is illustrated in FIG. 1.        
During application of the primary cathode material, the liquid phase of a suspension will enter the pores of the anode. If the particles in the suspension are larger than the pores, they will be prevented from entering the anode body and can buildup on the external surface of the anode. Thus external buildup on the anode after application of the slurry is somewhat dependent on the void volume (i.e. density) of the anode. Variations in local density of the anode can result in non-uniform coating, especially on the corners and edges of an anode.
It is also common that the mechanically press-formed anodes exhibit small protrusions, or “lips”, analogous to parting lines on injection molded plastic pieces where the valve metal powder formed into the clearance where tooling components meet. Practical tooling design dictates that the tooling joint occurs at the edges of the pellet. This situation exacerbates the previously discussed tendency of the cathode material to pull away from the edges of the pellet, making uniform coverage of a polymer coating over the entire anode body even more difficult.
The reliability of a solid electrolytic capacitor is also degraded due to differences in coefficients of thermal expansion between the anode bodies and encapsulates material. These mismatches lead to thermo-mechanical stresses on the anode/dielectric during surface mounting of the capacitor device onto a circuit board. These stresses are greatest at the corners and edges of the anode body. Capacitor manufacturers rely on the external cathode coatings of carbon and silver paint to reduce or distribute the stress, especially at high stress points like corners and edges of the anode. However, per the previously discussed anomaly of thin cathode layer coverage on corners and edges of the pellet, there is a direct relationship between the curvature of the corners and edges of the pellet and the reliability of the device.
Capacitor manufacturers have also employed rectangular prism anode designs with designated rounded or chamfered edges for various performance related reasons. For example, in order to reduce the thermo-mechanical stress on edges of surface mount devices after encapsulation, an anode with chamfered edges at the top of the anode was described by D. M. Edson and J. B. Fortin in a paper published in the Capacitor and Resistor Symposium in March 1994 entitled “Improving Thermal Shock Resistance of Surface Mount Tantalum Capacitors.” These authors used finite element analysis and failure site identification techniques to demonstrate that most failures which occurred during surface mounting were along the top edges of the anode (surface where the lead projects). A modified anode design, as depicted in FIG. 3, was reported to reduce the surface mount failure rate.
Anode bodies with 4 curved or chamfered edges, those parallel to the anode lead wire, have been observed in capacitors on the market. (See FIG. 4). These anode bodies are pressed in a die set that compacts in the same direction of the anode lead wire axis (termed “axial pressing”), which is typically in the long axis of the compact. By nature of the axial pressing process, greater flexibility in anode geometries of interest to capacitor manufacturers can be achieved, but the process has a number of associated drawbacks. Primarily, axially compacted pellets still exhibit sharp corners and/or edges at the ends of the pellet, regardless of profile that may contain rounded edges. Secondarily, the axial pressing process often leads to problems such as smearing/burnishing of the powder on the exterior pellet surface (FIG. 7) as it slides inside the die cavity during compaction and ejection process steps. Additionally, axially pressed anodes often exhibit a significant density gradient and porosity gradient in the direction of compaction. Powder smearing and high density regions make it more difficult for the liquid phase of a slurry or suspension to enter the pores of the anode evenly, exacerbating the problems of poor cathode coverage.
Another approach to improving corner coverage would be to eliminate the side edges through the use of cylindrical or obround anode geometries. However cylindrical anodes are volumetrically inefficient when used in industry standard case dimensions for surface mount product. Obround anodes are more volumetrically efficient, but pressing these anodes is generally done on an axial style press. This leads to the previously discussed anomalies of axially pressed pellets.
Axial leaded hermetically sealed solid electrolytic capacitors are extremely reliable capacitors. This is because the only heat introduced in soldering the device to the circuit board is to the leads, which is done on the opposite side of the circuit board from the part, thus the temperature rise inside the device is small, and damaging forces (mismatch in coefficients of thermal expansion) created by this process are minimal. Compared to the forces created in the solder process for surface mount capacitors (SMT) where the entire capacitor package is immersed into the high thermal profile of the solder, these forces should never create failures. This fact is born out in the recommended applications of these capacitors. Leaded capacitors may be used up to 80% of its nameplate voltage, whereas the SMT product is limited to 50% of its nameplate voltage.
A reliability issue for these leaded products is the susceptibility to mechanical forces created in handling of the parts. As loose pieces are handled, there is a potential of dropping the device, crimping the device, or pressing on the device, in which the internal damage may not be detectable. If the piece survives the initial electrical testing, the flaw created by the physical force can grow and become a circuit failure at a later point in time.